The present invention relates generally to direct current (DC) driver circuits. More particularly, this invention pertains to constant current DC driver circuits for light emitting diode (LED) light sources.
Light fixtures including a driver circuit packaged with LEDs are replacing incandescent and compact fluorescent light bulbs. However, LED light packages and fixtures need to be controlled for temperature, voltage, and current. These light fixtures may include housings in shapes other than standard incandescent bulb packages. An LED driver circuit is typically a constant current driver circuit that controls current by varying the duty cycle and/or the switching frequency of a DC-to-DC converter in the driver circuit. The DC-to-DC converter may be a flyback or buck converter. Switches in the DC-to-DC converter are controlled by a pulse width modulation (PWM) circuit to provide power from the DC-to-DC converter to the LEDs. Separately, a dimming circuit interprets variations in an input voltage to the light fixture, or a control signal to the dimming circuit, to provide a microprocessor in the driver circuit with a signal indicative of a dimming level. The microprocessor controls the PWM circuit to adjust operation of the DC-to-DC converter accordingly.
In operation, when the light fixture receives power, portions of the driver circuit (i.e., the PWM circuit, the microprocessor, the dimming circuit, safety circuits, etc.) initialize in an uncontrolled fashion. This can potentially lead to runaway operation, over-voltage conditions, over-current conditions, and a failure to start. Additionally, startup fault detection combined with controlled shutdowns and restarts are generally not possible without significant additional circuitry. Control of the PWM circuit by the microprocessor is granular and activation of, for example, safety circuits can cause the driver circuit to intermittently shut down entirely when no actual fault exists (e.g., when the light fixture is operating near a predetermined temperature limit).